Quantifying topological protection of light on a chip

Photonic topological insulators are currently at the forefront of on-chip photonic research due to their potential for loss-free information transport. Realized in photonic crystals, they enable robust propagation of optical states along domain walls. But how robust is robust? In order to answer this, we quantified photonic edge state transport using phase-resolved near-field optical microscopy, in close collaboration with our colleagues from the group of Kobus Kuipers in Delft. The findings provide a crucial step towards error-free integrated photonic networks. The results were published in the journal Light Science & Applications.

Topologically tailored photonic crystals have opened up the possibility for attaining robust unidirectional transport of photonic signals. Their unprecedented guiding capabilities – supporting unhindered transport around imperfections and sharp corners without the need for any optimization – are highly beneficial for efficient distribution of information through dense on-chip photonic networks at telecom wavelengths. However, transport properties of such topologically non-trivial states have so far only been inferred by transmission measurements, and quantification of robustness remains challenging.

In a new paper published in Light: Science & Applications, René Barczyk from our group together with Sonakshi Arora and Thomas Bauer from the group of Kobus Kuipers at Delft University of Technology, reports a rigorous robustness evaluation of photonic edge eigenstates at telecom wavelengths.

We fabricated a topological photonic crystal that consists of triangular holes perforating a silicon membrane. The crystal contains two different hole sizes, which gives the mathematical description of light waves propagating in the crystal a special structure, or ‘topology’. Then, when two crystals with different arrangements of the smaller and larger holes are brought together, the boundary between both crystals supports a light wave that has intriguing properties: it can flow unhindered around any sharp corner of the boundary. We visualized those photonic edge states in the photonic chip with a phase-resolved near-field optical microscope. The technique allowed us to separate forward propagating light from backward travelling waves with extreme sensitivity and thus perform local monitoring of back-scattering along the domain wall.

We further complemented their quantitative analysis of the topological edge waves with light waves propagating in an ordinary photonic crystal waveguide. In stark contrast to waves in the topological crystal, the light in the ordinary crystal shows significant loss across a ‘defect’, consisting of four sharp waveguide corners. The light waves are scattered to a back-reflected wave. The topological waves instead flowed around the sharp corners seemingly unhindered. We were able to conclude that the topological protection reduces the back-reflection from an individual corner by two orders of magnitude for all optical frequencies.

Experimental visualization of light propagation in a topological photonic crystal. The top shows a two-dimensional map of the light field flowing unhindered around four corners. It is measured with a scanning probe that detects the field just above the photonic crystal membrane in which it propagates. The bottom shows an electron microscope image of the structure, with the domain wall that guides the edge waves separating the blue and red domains of topological crystals. Images made by: Thomas Bauer and Sonakshi Arora